Geometry Unit 2: Reasoning and Proof
 Proof with numbered statements and reasons in
logical order.
 Write a two column proof for the following:
 If A, B, C, and D are points on a line, in the given order,
and AB = CD, then AC = BD.
 NOTE: The if part of the statement is the given part. The then
part it the section you must prove. Use a diagram to show the
given information.
 It is helpful to draw a diagram before you begin
your proof. Draw the diagram for the example
below:
 If A, B, C, and D are points on a line, in the given
order, and AB = CD, then AC = BD.
 If A, B, C, and D are points on a line, in the given
order, and AB = CD, then AC = BD.
 Start by writing the given and prove statements at the
top.
 Given: A, B, C, and D are points in a line in the order
given. AB = CD.
 Prove: AC = BD.
 Begin by creating two columns; a statement column
and a proof column.
 The first statement will ALWAYS be your given
statement with the reasoning being given.
 The continuing statements will be from your
reasoning from postulates, definitions, and
theorems.
 Segment, Angle, Ray, Line, Point, etc.
 Tick Marks
 Segments
 Angles
 Parallel
 Perpendicular
 Measure of Angles
 Which can you assume true?
 AD ≈ BC
 AB ≈ CD
 CD ≈ BC
 AB || CD
 AB ⊥ 𝐴𝐷
 ABCD is a square
 ABCD is a rectangle
 M<DCA = 45º
 M<CAB = 45º
 If A, B, C, and D are points on a line, in the given
order, and AB = CD, then AC = BD.
Statement
Reason
1. AB = CD
2. A, B, C, D are collinear in that
order
1. Given
2. Given
3. BC = BC
4. AC = AB + BC and BD = CD + BC
5. AB+ BC = CD + BC
3. Reflexive Property of Segments
4. Segment Addition Postulate
5. Addition Property of Equality
6. AC = BD
6. Substitution Property
 Given: BF bisects <ABC; <ABD ≈ <CBE.
 Prove: <DBF ≈ <EBF.
Statement
1.
2.
Reason
1. Given
2.
3.
4.
5.
3.
4.
5.
6.
7.
8.
9.
6.
7.
8.
9.
 Given: <A ≈ <B and <C ≈ <D.
 Prove: m<A + m<C = m<B + m<D.
Statement
1.
2.
3.
4.
Reason
1. Given
2.
3.
4.
 Given: A, B, C, and D are collinear and AB ≈ CD.
 Prove: AC ≈ BD.
Statement
1.
2.
Reason
1. Given
2.
3.
4.
5.
3.
4.
5.
6.
7.
8.
9.
6.
7.
8.
9.
 Given: <A and <B are supplementary angles and <
A and <C are supplementary angles.
 Prove: AC ≈ BD.
Statement
1.
2.
Reason
≈
1. Given
2.
3.
4.
3.
4.
5.
5.
6.
6.
 Given: <A and <B are supplementary angles and <
A and <C are supplementary angles.
 Prove: AC ≈ BD.
Statement
1.
2.
Reason
≈
1. Given
2.
3.
4.
3.
4.
5.
5.
6.
6.